Aristarchus of Samos, an early Greek astronomer (about 310 to 230 BC), was the first to suggest that the Earth revolved around the Sun, rather than the other way around. He gave the first estimate of the distance of the Moon (section (8c)), and it was his careful observation of a lunar eclipse--pin-pointing the Sun's position on the opposite side of the sky--that enabled Hipparchus, 169 years later, to deduce the precession of the equinoxes).

Except for one calculation--an estimate of the distance and size of the Sun--no work of Aristarchus has survived. However, one could guess why he believed that the Sun, not the Earth, was the central body around which the other one revolved. His calculation suggested that the Sun was much bigger than the Earth--a watermelon, compared to a peach--and it seemed unlikely that the larger body would orbit one so much smaller.

Here we will develop a line of reasoning somewhat like the one Aristarchus used (for his actual calculation, see reference at the end). Aristarchus started from an observation of a lunar eclipse (section (8c)). At such a time the Moon moves through the Earth's shadow, and what Aristarchus saw convinced him that the shadow was about twice as wide as the Moon. Suppose the width of the shadow was also the width of the Earth (actually it is less--see below). Then the diameter of the Moon would be half the Earth's.

Aristarchus next tried to observe exactly when half the moon was sunlit. For this to happen, the angle Earth-Moon-Sun (angle EMS in the drawing here) must be exactly 90 degrees.

Knowing the Sun's motion across the sky, Aristarchus could also locate the point P in the sky, on the Moon's orbit (near the ecliptic), which was exactly 90 degrees from the direction of the Sun as seen from Earth. If the Sun were very, very far away, the half-moon would also be on this line, at a position like M' (drawn with a different distance scale, for clarity).

Aristarchus estimated, however, that the direction to the half-Moon made a small anglea with the direction to P, about 1/30 of a right angle or 3 degrees.

As the drawing shows, the angle ESM (Earth-Sun-Moon) then also equals 3 degrees. If Rs is the Sun's distance and Rm the Moon's, a full 360° circle around the Sun at the Earth's distance has length of 2pRs (p = 3.14159...). The distance Rm = EM is then about as long as an arc of that circle, covering only 3° or 1/120 of the full circle. It follows that

Rm = 2pRs/120 ~ Rs/19
Therefore
Rs/Rm ~ 19

making the Sun (according to Aristarchus) 19 times more distant than the Moon. Since the two have very nearly the same size in the sky, even though one of them is 19 times more distant, the Sun must also be 19 times larger in diameter than the Moon.

If now the Moon's diameter is half the size of the Earth's, the Sun must be 19/2 or nearly 10 times wider than the Earth. The effect described in the figures of section (8c) modifies this argument somewhat (details here), making the Earth 3 times wider than the Moon, not twice. If Aristarchus had observed correctly, that would make the Sun's diameter 19/3 times--a bit more than 6 times--that the Earth.

Actually, he had not! His method does not really work, because in actuality the position of the half-Moonis very close to the line OP. The angle p, far from being 3 degrees, is actually so small that Aristarchus could never have measured it, especially without a telescope. The actual distance to the Sun is about 400 times that of the Moon, not 19 times, and the Sun's diameter is similarly about 400 times the Moon's and more than 100 times the Earth's

But it makes no difference. The main conclusion, that the Sun is vastly bigger than Earth, still holds. Aristarchus could just as well have said that the angle p was at most 3 degrees, in which case the Sun was at least 19 times more distant than the Moon, and its size at least 19/3 times that of Earth. In fact he did say so--but he also claimed it was less than 43/6 times larger than the Earth (Greeks used simple fractions--they knew nothing about decimals), which was widely off the mark. But it makes no difference: as long as the Sun is much bigger than the Earth, it makes more sense that it, rather than the Earth, is at the center.

Gilbert was a staunch supporter of Copernicus (see section #9c), but it is interesting to note that he still quotes the result of Aristarchus (Gilbert's "Book 6", section 2, about 2/3 through the section), writing "The Sun in its greatest eccentricity has a distance of 1142 semi-diameters of the Earth." Aristarchus estimated the Sun's distance to be slightly below 20 times that of the Moon, which was at a distance of about 60 Earth radii, and 20x60 = 1200, close to Gilbert's figure. In 1800 years, no one had checked that result!

The introduction to Gilbert' book was written by Edward Wright, who used that value to derive the velocity of the Sun, if it were to circle the Earth every 24 hours, ariving at a speed so high that he considered it impossible:

Then whether it would seem more probable, that the aequator of the terrestrial globe, in a single second (that is, in about the time in which one walking quickly will be able to advance only a single pace) can acomplish a quarter of a British mile (of which sixty equal one degree of a great circle on the Earth), or that the aequator of the primum mobile in the same time should traverse five thousand miles with celerity ineffable... swifter that the wings of lightning, if indeed they maintain the truth which especially assail the motion of the earth.

This is the same argument Aristarchus could have made, and probably did.

For advanced students:

Note: The actual calculation by Aristarchus was more complex and less transparent. See A. Pannekoek: A History of Astronomy, Interscience, 1961, p. 118-120 and Appendix A.

Teachers using this web page will find a related lesson plan at Larist.htm It belongs to a set of lesson plans whose home page is at Lintro.htm.